Toxic Substances Hydrology Program
Biofilms in Streams Help Create Daily Variations in Metal Concentrations
U.S. Geological Survey (USGS) scientists and their colleagues have demonstrated that the large daily changes in zinc concentrations observed in mining-affected streams are in some cases caused by biofilms. Biofilms are layers of greenish-brown slime commonly found on rocks and plants in streams. They are composed of microscopic bacteria, fungi, diatoms, and other algae. The scientists conducted a series of laboratory and field experiments that documented biofilms absorbing zinc from stream water during daylight hours and then releasing zinc to the stream during the night. This alternating absorption and release can create the daily changes in zinc concentrations observed in streams. Scientists, regulators, land managers, and reclamation teams working in metals-contaminated streams need to be aware of this daily metal cycling to ensure that monitoring and assessments of aquatic toxicity are accurate. The highest zinc concentrations (and therefore, perhaps the most toxic conditions) in stream water might occur only at night, and the greatest exposure to aquatic invertebrates (insects and worms), which feed on biofilms, might only occur during daylight hours.
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